1. Field of the Invention
The present invention relates to a semiconductor integrated circuit (hereinafter referred to as an IC) having an internal voltage generating circuit capable of providing an internal voltage having little dependency on a variation of an external power supply voltage.
2. Description of the Related Art
In a currently used dynamic random access memory (DRAM), it is desired that a voltage be generated by an IC itself, rather than using an external power supply voltage. Thereby, even if a plurality of voltage levels are required in the IC, only a single external power supply voltage may be supplied to the IC. In a modern DRAM, a single external power supply voltage is employed, and other necessary voltages are generated within the IC. The external power supply voltage is determined according to the breakdown voltage of the IC, the use of the IC, etc. It is inevitably required to reduce the external power supply voltage, coping with of an increase in integration density, a decrease in power consumption, and an electric cell-powered operation.
On the other hand, a voltage required within the IC is selected in consideration of the thickness of an oxide film of an MOS transistor used in the IC, power consumption, data write potential in memory cells, and reliability and so on. According to the natural scaling rule, it is supposed that the power supply voltage is scaled similarly. Although a decrease in voltage is required both for the external power supply voltage and internal power supply voltage, the required voltage is not necessarily equal. In order to ensure that the IC operates in a wide range of external power supply voltages, it is desired that an internal voltage with low dependency on the external power supply voltage be generated. Conventional internal voltage generating circuits include one using a charge pump and one using a bootstrap in a case where a potential higher than an external power supply voltage is generated, and also one using a charge pump and one using a voltage-decrease circuit in the case of a potential lower than an external power supply voltage is generated.
In the prior art, an internal voltage-decrease circuit generates a voltage with low dependency on the external power supply voltage, thereby ensuring highly reliable operation in a wider range of operation power supply voltages. In this system, however, the range of internal voltages that can be set is considerably limited due to the above-mentioned lowering of the external power supply voltage. In particular, when the external power supply voltage is low, the operation margin of the IC is decreased.
On the other hand, in a system wherein voltage is boosted by a boost circuit over the entire range of normal operation power supply voltages of the IC, when the external power supply voltage is high, the IC may be damaged or the reliability of the IC is degraded due to a decrease in thickness of oxide films of MOS transistors used in the IC. Furthermore, in the prior art in which the relationship between a high level and a low level of the external power supply voltage is reversed, the same problem as above occurs. The above problem applies not only to DRAMs but also to other types of high integration density semiconductor ICs.
In order to ensure the operation of the IC over the wide range of external power supply voltages, as mentioned above, it is desirable to generate an internal voltage with low dependency on the external power supply voltage.
Suppose an internal voltage with low dependency on the external power supply voltage is generated by using a voltage-decrease circuit which generates an internal voltage lower than a high-voltage-side power supply voltage Vcc of the external power supply voltage. In this case, if the high-voltage-side power supply voltage is varied towards a narrower range of variation, the input voltage becomes the internal voltage as it is, and it is not sufficiently boosted. As a result, the operation margin of the IC may occur.
Suppose an internal voltage with low dependency on an external power supply voltage is generated by using a voltage boost circuit which generates an internal voltage higher than a high-voltage-side power supply voltage Vcc of the external power supply voltage. In this case, if the high-voltage-side power supply voltage is varied towards a wider range of variation, the input voltage becomes the internal voltage as it is, and it is not sufficiently decreased. As a result, an excess voltage is generated, and the IC may be damaged or the reliability of the IC may be degraded.
The above problem will occur when the voltage Vcc is boosted or decreased, for example, in the case where the internal voltage is applied to a gate of an N-channel transfer transistor.
Inversely, when the internal voltage is applied to a gate of a P-channel transfer transistor, the relationship between the high-voltage-side power supply voltage Vcc and low-voltage-side power supply voltage Vss (ground potential) of the external power supply voltage is reversed. Specifically, suppose an internal voltage with low dependency on an external power supply voltage is generated by using a voltage boost circuit which generates an internal voltage higher than a low-voltage-side power supply voltage Vss. In this case, if the high-voltage-side power supply voltage is varied towards a narrower range of variation, the potential difference between the internal voltage and high-voltage-side power supply voltage becomes insufficient, and the operation margin of the IC is decreased.
On the other hand, suppose that an internal voltage with low dependency on an external power supply voltage is generated by using a voltage-decrease circuit which generates an internal voltage lower than a low-voltage-side power supply voltage Vss of the external power supply voltage. In this case, if the high-voltage-side power supply voltage is varied towards a wider range of variation, the potential difference between the internal voltage and external power supply voltage becomes too great, and the IC may be damaged or the reliability of the IC may be degraded.
In any case, considerable limitations are put on the set level of the internal voltage, and the range of operation power supply voltages is limited and the reliability is degraded.